Chapter 11.4, Problem 50ES

If a force F is applied to an object at a point Q, then the line through Q parallel to F is called the line of action of the force. We defined the vector moment of F about a point P to be $\stackrel{\to }{PQ}\times \text{F}$ . Show that if $Q^{\prime }$ is any point on the line of action of F, then $\stackrel{\to }{PQ}\times \text{F=}\stackrel{\to }{PQ}\times \text{F;}$ that is, it is not essential to use the point of application to compute the vector moment—any point on the line of action will do.